Device for diverting fluid from a pipeline

ABSTRACT

There is provided a device for diverting liquid from a pipeline, and particularly for use with multi-phase flows where separation of liquid from gas is required. The device comprises a first conduit ( 50 ), a second conduit ( 52 ) connected across at least part of the first conduit, and means ( 80 ) for controlling flow of liquid from the first conduit through the second conduit by use of hydrostatic pressure. The first conduit is provided with an inlet ( 56 ) at substantially right angles to the first conduit. The second conduit further comprises an annulus ( 58 ) extending inwards from an inner wall of the first conduit and a lip ( 60 ) extending upwards from an inner circumference of the annulus. A number of embodiments of the device are described. The device is also suitable for fitting to flow meters to extend their range of operation.

FIELD OF THE INVENTION

[0001] This invention relates to a device for diverting fluid from apipeline and is particularly concerned with separating oil and waterfrom a multi-phase flow of gas, oil and water.

BACKGROUND TO THE INVENTION

[0002] In the oil industry, the water-liquid-ratio (wlr) is an importantmeasurement of the output flowing from a well and for wells producing athree-phase flow (gas, oil and water) separator systems are used toseparate the gas, oil and water into individual streams to simplify thewlr measurement. However separator systems are heavy, bulky, costly andprone to failure and obtaining three separate streams for each componentcan be complicated and costly.

[0003] Where the liquid, i.e. water and oil, is not separated andmetering is performed on the multi-phase flow, the measurement of thewlr can be very difficult, particularly when the volume fraction of gasin the line is high. For example, where the wlr is 10% in a multi-phaseflow with 90% gas, then 1% of the total volume is water, 9% is oil and90% is gas. Measurement of the wlr in a multi-phase flow requiresdetecting the presence of the 1% by volume of water, whereas if all thegas is removed leaving an oil-water flow, measurement of the wlrrequires detecting the presence of the 10% by volume of water. Removinggas from the multi-phase stream to produce a liquid-rich streamtherefore makes measurement of the wlr easier.

[0004] Hydrocyclones are used to produce a liquid-rich stream from amulti-phase flow. However these systems tend to be large and result inthe liquid and gas phases travelling in opposite directions which cancause problems with pipe layout.

[0005] It is an aim of the present invention to produce a device forobtaining a liquid-rich stream without the disadvantages associated withthe prior art, and also aims to provide a device for retrofitting toexisting flow meters to increase their range of operation.

SUMMARY OF THE INVENTION

[0006] In accordance with one aspect of the present invention, there isa device for diverting liquid from a pipeline, the device comprising afirst conduit, a second conduit connected across at least part of thefirst conduit, and means for controlling flow of liquid from the firstconduit through the second conduit by use of hydrostatic pressure.

[0007] Such a device is particularly applicable for surface pipelinestransporting gas-rich flows from wells, the flow being of around 90%gas, with the remaining percentage by volume consisting of oil andwater. The device is suitable for use as a flowmeter when used incombination with appropriate gauges, and also is such as to allowsampling as liquid can be tapped off separately to gas. The device isalso intended for retrofitting to existing flowmeters to increase therange over which flows can be measured, with the device acting toprovide an offset to these meters, or the device can be incorporatedinto the flowmeter during manufacture.

[0008] In use, the device is typically connected to a pipelinetransporting a multi-phase flow which is predominantly made up of gas,with the first conduit arranged to be substantially vertical relative tothe earth's surface.

[0009] Therefore the first conduit in use is preferably connectedbetween two sections of an existing pipeline. This particularly requiresthe pipeline flow to be stopped whilst the pipeline is cut to allow theplacing of the device.

[0010] The means for controlling flow of liquid from the first conduitthrough the second conduit is preferably provided by an S-shaped sectionwithin the second conduit. Thus the second conduit may incorporate themeans for controlling the flow of liquid. By using an S-shaped bend andarranging the second conduit to provide a bypass route across a lengthof the first conduit, the pressure drop across the bypassed length ofthe first conduit will be balanced by a difference in the height offluid in the two curved sections forming the S-shaped bend. As a result,if more liquid is introduced into the second conduit, the level of fluidin the S-shaped bend alters to remain in equilibrium with the pressuredrop across the length of the first conduit, as a result of hydrostaticpressure, and thus new fluid introduced into the second conduit willforce fluid out of the S-bend and into a return section of the secondconduit thereby to return to the first conduit.

[0011] The second conduit may further comprise a collecting means placedat least partly within the first conduit, and thus the second conduitpreferably further comprises an annulus extending inwards from an innerwall of the first conduit and a lip extending upwards from an innercircumference of the annulus, and acting to trap liquid travelling alongthe walls of the first conduit and direct liquid into the secondconduit.

[0012] The second conduit may comprise an elongate lip attached to theannulus or collar, with the elongate lip forming a baffle plate to actto separate liquid from the gas flow.

[0013] Preferably the first conduit is provided with an inlet atsubstantially right angles to the first conduit. This tangential inletensures that in use fluid passing into the first conduit gains a certaindegree of centrifugal force to further assist with separation of liquidcomponents from the gas.

[0014] The collecting means may further comprise receptacle means incommunication with the first conduit and the annulus. This allows avolume of liquid to be stored before the liquid enters the S-shapedsection, and so provides time for gas inadvertently trapped in theliquid to escape the liquid whilst it is held in the receptacle meansprior to entry into the S-shaped section, the gas then returning to thefirst conduit.

[0015] The receptacle means is preferably placed adjacent to the annulusand the first conduit, and connected thereto by first and secondpassages.

[0016] Alternatively the receptacle means surrounds the annulus and atleast part of the first conduit with two spaced apart apertures in anencased wall of the first conduit providing communication between thereceptacle means, annulus and first conduit.

[0017] The second conduit may further comprise an elongate sectionextending from an end of the S-shaped section furthest from the firstconduit, the elongate section providing a generally downward path andjoining with the first conduit at a distance below an inlet to thesecond conduit.

[0018] The second conduit may comprise a delay section leading from thecollecting means and joining with a first end of the S-shaped section,the delay section comprising a hollow cylinder of tapering cross sectionwhich is wound around the first conduit to form a spiral.

[0019] The device is suitable for use with surface pipes, but may beadapted for use on a vertical well pipe such as a borehole.

BRIEF DESCRIPTION OF DRAWINGS

[0020] The invention is now described, by way of example, with referenceto the following drawings in which:

[0021]FIG. 1 shows a section through a first embodiment of a device inaccordance with the present invention;

[0022]FIG. 2 shows a sectional view on the line II-II of FIG. 1;

[0023]FIG. 3 shows a section through a second embodiment of a device inaccordance with the present invention;

[0024]FIG. 4 corresponds to FIG. 3 and is used to explain operation ofthe device;

[0025]FIG. 5 is a graph illustrating the amount of liquid extracted as apercentage of liquid input for the device shown in FIG. 3;

[0026]FIG. 6 shows a section through of a third embodiment of a devicein accordance with the present invention;

[0027]FIG. 7 shows a section through a fourth embodiment of a device inaccordance with the present invention;

[0028]FIG. 8 shows a section through a fifth embodiment of a device inaccordance with the present invention; and

[0029]FIG. 9 shows a schematic diagram of a flowmeter incorporating adevice in accordance with the present invention.

DESCRIPTION

[0030] A device 10 in accordance with the present invention isillustrated in FIG. 1. Typically the device is inserted into a surfacepipeline carrying a multi-phase flow of gas, oil and water from a well.To insert the device, the flow in the pipeline is stopped, and thepipeline cut and modified so that an upstream portion of pipeline feedsinto an inlet 12 of the device with a downstream portion of the pipelineconnected to an outlet 14 of the device. The device is placed at rightangles to ground level.

[0031] The device 10 is made from metal and comprises a first conduit 20and a second conduit 22. The first conduit 20 has a circularcross-section of typically the same size as the cross-section of thepipeline, whilst the second conduit 22 has generally a substantiallysmaller circular cross-section than the first conduit. The walls of thefirst and second conduits are of a suitable thickness to withstand thepressures associated with multi-phase production flows, and thus aretypically of a thickness that will withstand 5000 psi.

[0032] The second conduit 22 is connected across a length L of thevertical first conduit 20 so providing a path along which liquid can betemporarily diverted from the main conduit, entering inlet 24 beforereturning to the main conduit at outlet 26. The second conduit 22comprises a collecting means 30 joined to the inlet 24 and which sitswithin the first conduit 20, a first elongate section 32 of pipeattached between the inlet 24 and a first end 34 of an S-shaped section36, a second elongate section 38 joined to a second end 40 of theS-shaped section, and a downwardly slanting section 42 leading from thesecond elongate section to the outlet 26 and joining to the firstconduit 20. The collecting means 30 comprises an annulus 44, of the sameouter diameter as the first conduit, and a lip 46 extending upwards froman inner edge 50 of the annulus 44.

[0033]FIG. 2 shows a sectional view along line II-II of FIG. 1 fromwhich can be seen the cross-section of the second conduit 22, excludingthe collecting means, and the cross-section of the first conduit 20 arecircular, with the diameter of the first conduit 20 being substantiallygreater than the diameter of the second conduit 22.

[0034] When a multi-phase flow travels in a pipeline, the liquid in theflow, i.e. oil and water, predominantly travels along the walls of thepipeline as a result of frictional effects. Thus by placing an annulus44 with a lip 46 within the first conduit 20, the liquid portion of thegas-liquid flow is channelled into the second conduit 22. In theembodiment shown in FIG. 1, some gas will pass with this liquid into thesecond conduit 22, but, as will be explained later, due to the residencetime of liquid within the S-shaped section 36, much of the gas willreturn to the first conduit 20.

[0035] A second embodiment of the device is shown in FIG. 3, andcomprises a main conduit 50 and a secondary conduit 52 incorporating amore complex collecting means 54 than that of the first embodiment.Instead of flow from the pipeline travelling down into the firstconduit, i.e. with gravity, a tangential inlet 56 to the main conduit 50is provided. The second collecting means 54 comprises an annulus 58 withan elongate lip 60 which extends up beyond the tangential inlet 56 so asto act as baffle plate, and a substantially enclosed cylinder 62. Thecylinder has a lower inlet 64 which joins with the first conduit 50 andso connects to the annulus 58 as well, and an upper inlet 66 placedfurther up the wall of the first conduit. The remainder of the secondconduit comprises a first elongate section 70, an upper end 72 of whichextends up and into the cylinder 62 and is open to receive liquid, witha lower end 74 attached to a first end of an S-shaped section 80. Aflared elongate section 82 joins to a second end of the S-shaped section80 and leads into a downwardly slanting section 84 which in turnconnects with a horizontal portion 86 of pipe meeting the first conduit50 at a position below the collecting means. An anti-siphon line 90 isprovided between the flared section 82 and an uppermost end 92 of thefirst conduit 50 so as to ensure that automatic siphoning of the liquidthrough the system does not occur. The dimensions of the device areapproximately 1200 mm (high)×500 mm×500 mm.

[0036] This embodiment has enhanced gas rejection over the firstembodiment as the passage time of the liquid through the device isincreased due to the increased volume of the collecting means. Inaddition, the tangential inlet imparts a certain degree of centrifugalforce to the fluid as it enters the first conduit and this produces aswirling effect in the flow which assists with separation of liquid fromgas. The baffle plate also acts to increase separation of liquid fromgas as, when the multi-phase flow hits the baffle plate, the passage ofliquid is abruptly halted causing the liquid to fall to the base of thecollecting annulus. However the gas is not so affected and passes alongthe length of the first conduit.

[0037] The operation of the device will now be described with referenceto FIG. 4, which uses common reference numerals to FIG. 3 whereappropriate. The device uses the principle of hydrostatic pressure toprovide a controllable passage of liquid through the system, eventuallyto return to the first conduit. The device thus avoids the need for anymoving parts or any external control for the device to operate. When amulti-phase flow enters the first conduit 50 via the tangential inlet56, liquid incident on the baffle plate 60 falls down to the collectingannulus 58 and, due to the interconnection of the annulus, cylinder 62and S-shaped bend 80, will pass into all these parts of the secondconduit. Gas in the multi-phase flow is largely unaffected by thepresence of the baffle plate 60 and generally will simply continueflowing along the length of the first conduit, although some gas will betrapped in the liquid falling into annulus 58.

[0038] A pressure drop exists in the first conduit 50 due to gravity,with gas at the upper end 92 of the conduit being at a lower pressure P₁than gas at a lower end 94 of the conduit which is at pressure P₂. Thepressure difference, P₁-P₂, of around 100 mbar is balanced by the headof liquid in the S-shaped bend 80, i.e. the pressure exerted by theliquid h_(liquid)−h₀. Thus in the equilibrium position where fluid hasbeen introduced into the second conduit but where, for example, flow hasthen stopped, the height of liquid in the S-shaped bend is greater thanthe height of the liquid in the collecting means by an amount thatbalances the pressure difference.

[0039] As more liquid is introduced into the collecting portion of theconduit, the system moves out of equilibrium. Thus the level of liquidin the cylinder and annulus will be such that the head of liquid doesnot balance the pressure drop ΔP. The system will act to restore theequilibrium state and thus increase the level of fluid in the S-bend toh_(level) so as to ensure that the head balances the pressure drop.Liquid is thus forced up and out of a vertical portion 96 of the S-bendand into the flared portion 82 to return to the first conduit, as thesystem continuously acts to restore equilibrium as liquid flows into thesecond conduit. The maximum liquid extraction flow rate is a function ofthe dimensions of the device, but for a device of dimension 1200 mm×500mm×500 mm is typically 8 m³ an hour.

[0040] To explain in more detail, the equilibrium state is thus when noliquid is extracted and the hydrostatic head, ρg(h_(Level)−h₀) isbalanced by the pressure drop P₁-P₂:

ΔP=P ₁ −P ₂ =ρg(h _(Level) −h ₀)  (1)

[0041] where ρ is the liquid density, h_(Level) is the greatest heightof liquid in S-shaped section 80 of diameter d, and h_(liquid) is theheight of liquid in the cylinder which has diameter D.

[0042] For a liquid velocity of ν₁ min diameter D, the velocity ν₂ indiameter d is $\begin{matrix}{v_{2} = {v_{1}\frac{D^{2}}{d^{2}}}} & (2)\end{matrix}$

[0043] When h_(Liquid)>h₀ then liquid flows through the device and witha liquid velocity in diameter D of ν₁, the liquid velocity ν₂ indiameter d, can be written as $\begin{matrix}{{v_{2} = \sqrt{\left( \frac{2}{\rho} \right)\frac{\left( {{\Delta \quad P} - P_{Losses} - {\rho \quad {g\left( {h_{Level} - h_{Liquid}} \right)}}} \right)}{\left( {1 - \frac{d^{4}}{D^{4}}} \right)}}}{where}} & (3) \\{P_{Losses} = \frac{2\quad f\quad \rho \quad v_{2}^{2}L_{Losses}}{d}} & (4)\end{matrix}$

[0044] and

f=aRe ^(−b) Blasius formula, a=0.079, b=0.25  (5)

[0045] and $\begin{matrix}{{Re} = \frac{\rho \quad v_{2}d}{\eta_{liquid}}} & (6)\end{matrix}$

[0046] where P_(Losses) is the pressure loss in diameter d, L_(Losses)the equivalent straight pipe length diameter d, and η_(liquid) theviscosity of the liquid.

[0047] When ν₂=0 then P_(Losses)=0, and ΔP is given by equation (1). h₀should be chosen to be large enough so that no liquid enters the liquidleg, in which case $\begin{matrix}{{v_{2} = \sqrt{\left( \frac{2}{\rho} \right)\frac{\left( {{\rho \quad {g\left( {h_{Liquid} - h_{0}} \right)}} - P_{Losses}} \right)}{\left( {1 - \frac{d^{4}}{D^{4}}} \right)}}}{{{{For}\quad v_{2}} > 0},{{{{then}\quad P_{Losses}} < {\rho \quad {g\left( {h_{Liquid} - h_{0}} \right)}}} = 0}}} & (7)\end{matrix}$

[0048] The maximum value of ν₂ (or equivalently the maximum liquidextracted=ν₂πd²/4) is driven by h_(Liquid) and this determines the totalheight of the device.

[0049] Thus if there is only gas in the main flow line then theequilibrium state is when the hydrostatic pressure difference betweenh_(liquid) and h₀ equals the pressure difference between the top of thefirst conduit and where the second conduit returns to join the firstconduit. If the hydrostatic pressure of the liquid head is less than ΔP,then fluid will flow over the top bend of the S-shaped section andreturn to the main flow line via sections 82, 84, 86.

[0050] This system is self regulating in that liquid will only flow outof the S-bend section when liquid is in the annulus, cylinder andS-shaped section and the hydrostatic head, h, is too small to balancethe pressure difference ΔP. Hence a heavy liquid phase will flow throughthe device, if there is only gas in the main flow line there will be noflow through the device, and the device rejects gas.

[0051] As mentioned previously, some gas will be trapped with the liquidwhen it is collected from the first conduit, and to ensure that the wlrmeasurement is easy to perform, as much gas as possible needs to bereturned to the main conduit. A delay time, or lag, before liquid entersthe S-shaped section is desirable so that gas caught within the liquidcan escape.

[0052] There are a variety of ways of producing such a delay, with thesecond embodiment achieving this by increasing the volume of liquidwaiting to pass into the S-shaped section.

[0053] The increased residence time of the liquid in the collectionmeans allows gas bubbles trapped within the liquid to have an extendedtime in which to rise to the surface of the liquid and return to thefirst conduit by means of the first conducting passageway 66. There areother ways of increasing the residence time, and these are discussedwith reference to FIGS. 6 and 7. Ideally the residence time is around 5s or such that the time for gas to rise to the surface of liquid in thecollection means is less than the time for fluid to pass from thecollection means to the S-shaped bend. The residence time needed dependson the distance the gas has to travel through the liquid to reach aliquid-gas interface and the velocities of the gas and liquid phases.

[0054] The devices discussed herein selectively divert liquid from amulti-phase flow so that the wlr can readily be measured, without alarge proportion of gas being associated with the liquid and interferingwith the measurement. The devices have many uses in that they allowdirect sampling of the liquid by placing valves at positions A and B,and easy measurement of liquid and gas flow rates by placing a gas flowmeter at position C and a liquid flow and/or wlr meter at position D.The devices can also be used as a sandtrap by placing a valve at E todraw sand out of the base of cylinder 62, and the devices can be used toprovide liquid removal from a flow by pumping liquid out of the devicefrom any point in return path 82, 84, 86 before the liquid returns tothe main conduit. The device can also be used as a compact separator ofliquid for multi-phase flows.

[0055] In situations where a representative liquid sample is requiredthe collection means is positioned in an area of high mixing of gas andliquid.

[0056] The devices can also be used for measurement of oil shrinkage,cleaning of the system by fluid injection, and calibration of any meterpositioned at D by injecting fluids of known properties at knownvelocities.

[0057] Local heating can also be used to increase the flow of viscousfluids through the device.

[0058] Enhanced liquid removal can be achieved by careful design of theflow conditions upstream of the device and by positioning two or moredevices in series.

[0059] With a device such as shown in FIG. 3, it is possible to extractaround 90% of the liquid in a liquid gas flow. This is illustrated bythe graph of FIG. 5 which plots the “liquid extracted” against “theliquid input into the first conduit” for a variety of different waterliquid ratios ranging from glr (gas volume rate/liquid flow rate) lessthan 10 and gvf (gas volume rate/total volume) less than 0.91, up to glrin the range of 200-1000 and gvf in the range 0.995-0.999. The liquidextracted has less than 1% gas entrained.

[0060]FIG. 6 shows a further embodiment of a device in accordance withthe invention where a cylinder 100 surrounds a portion of a firstconduit 102 with upper 110 and lower 112 apertures in plate 114providing communication paths for gas and liquid between a collectingannulus 116 and the first conduit 102.

[0061]FIG. 7 illustrates another embodiment of the present invention,where to increase residence time of fluid in the device, a taperingcross-section pipe 120 is wound around a first conduit 122 to lead intoan S-shaped bend 124. The diameter of this pipe 120 and the pitch of thewinding are such that the flow in this pipe is stratified with theliquid on the lower surface of the pipe. In this case, any gas in theliquid has to travel a distance equal to the thickness of the liquidstratified layer before exiting via the collecting means 126. Having apipe with a decreasing diameter enhances this effect. This ensures thatthe liquid collected within the S-bend is to a large extent gas free.

[0062] A further embodiment is illustrated at FIG. 8, this being adevice for use in an operational well 130 with fluid flowing up tosurface as shown by arrow 132. By providing low pressure at one end 134of the device, liquid collection can be achieved in a similar manner asaforesaid.

[0063] A device in accordance with the present invention can also beused to modify existing flowmeters so as to extend their range ofoperation, and this is shown in FIG. 9.

[0064] Multi-phase flowmeters are used in the oil industry to measurethe flow rates of oil, water and gas in a pipeline without separatingthe phases. These meters have an operational range with a lower limitthat can generally only be modified by a change in dimensions or by useof additional meters. One way of lowering the operating range of amulti-phase meter therefore involves a second multi-phase meter inseries with the first. In the cases where the oil, water and gas phasesare separated into individual streams, lowering the range involvesadding additional meters to each flow line: this is costly as the numberof meters needed is doubled and each meter is expensive. However inaccordance with another aspect of the present invention, a device ingenerally the same form as that discussed previously is fitted to,either on manufacture or by retrofitting, a multi-phase meter so as toincrease flow by a known amount that allows the meter to function overan increased operating range.

[0065] Such a modified flow meter 138 is illustrated schematically inFIG. 9. The basic flow meter is described in Atkinson, I., Berard, M.B-V Hanssen, G Segeral: “New Generation Multiphase Flowmeters fromSchlumberger and Framo Eng.AS.,” 17^(th) International North Sea FlowMeasurement Workshop, Oslo, Norway, October 1999. A multi-phase flow 140is fed into an input 142 of the flowmeter and passes through the meterto outlet 144. A device 150 in accordance with the invention, such asthat depicted in FIG. 3, is inserted in the outlet, or downstream, pathof the meter and the device used to divert liquid from the multi-phaseflow, the liquid passing along path 152 to a liquid storage tank 154.Any gas contained in the liquid is returned along line 156 to the outletpath. The liquid in the liquid storage tank 154 is fed back along line158 to the inlet 142, and upstream of the metering section 159, by pump160, and the amount of liquid fed back is monitored by liquid flow meter162.

[0066] This increases the total liquid flow through the multi-phasemeter by a measured amount such that the flow through the meter iswithin the original operating range. The actual flow in the mainpipeline is computed from the difference between the flow measured bythe multi-phase meter and that measured by the meter in the liquid line.The flow ‘returned’ is only liquid phases to simplify the metering andpumping operation as then metering can be performed with a liquid meterand standard liquid pump. The accuracy of the liquid flow rate will bedecreased slightly as a result of using two meters, for example, if theaccuracy of the multi-phase meter is ±5% and the liquid meter is ±2%then the final accuracy of the combined meter will be ±5.4% (using RMSmethod).

[0067] The size of the tank 154 depends upon the efficiency of thedevice 150, the liquid volume rate required to be pumped to get themulti-phase meter within its operating range, and the measurement timeof the multi-phase meter.

[0068] The above-described embodiments are illustrative of the inventiononly and are not intended to limit the scope of the present invention.

1. A device for diverting liquid from a pipeline, the device comprising:a first conduit; a second conduit connected across at least part of thefirst conduit; and an assembly for controlling flow of liquid from thefirst conduit through the second conduit by use of hydrostatic pressure.2. A device according to claim 1, wherein the assembly for controllingflow of liquid from the first conduit through the second conduitcomprises an S-shaped section within the second conduit.
 3. A deviceaccording to claim 1 or claim 2, wherein the first conduit is providedwith an inlet at substantially right angles to the first conduit.
 4. Adevice according to any of the preceding claims, wherein the secondconduit further comprises a collector placed at least partly within thefirst conduit.
 5. A device according to claim 4, wherein the collectorcomprises an annulus extending inwards from an inner wall of the firstconduit and a lip extending upwards from an inner circumference of theannulus.
 6. A device according to claim 5, wherein the lip is elongateand extends upwards to an inlet of the first conduit, so as to act as abaffle plate.
 7. A device according to any of claims 4 to 6, wherein thecollector further comprises a receptacle in communication with the firstconduit and the annulus.
 8. A device according to claim 7, wherein thereceptacle is placed adjacent to the annulus and the first conduit, andconnected thereto by first and second passages.
 9. A device according toclaim 7, wherein the receptacle surrounds the annulus and at least partof the first conduit with two spaced apart apertures in an encased wallof the first conduit providing communication between the receptacle,annulus and first conduit.
 10. A device according to any of claims 2 to9, wherein the second conduit further comprises an elongate sectionextending from an end of the S-shaped section furthest from the firstconduit, the elongate section providing a generally downward path andjoining with the first conduit at a distance below an inlet to thesecond conduit.
 11. A device according to any of claims 4 to 6, whereinthe second conduit comprises a delay section leading from the collectorand joining with a first end of the S-shaped section, the delay sectioncomprising a hollow cylinder of tapering cross section which is woundaround the first conduit to form a spiral.
 12. A device according to anyof the preceding claims, wherein the first conduit is a borehole.
 13. Adevice according to any of the preceding claims, wherein an anti-siphonline connects the second conduit with the first conduit.
 14. A flowmeter fitted with a device according to any of the preceding claims.